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R. G. Herb Condensed Matter Seminar
Small Josephson Arrays as Protected Qubits
Date: Wednesday, November 18th
Time: 11:00 am - 12:00 pm
Place: 5310 Chamberlin
Speaker: Michael Gershenson, Rutgers University
Abstract: Within the past twenty years, there has been a growing realization that superconducting Josephson circuits can behave as macroscopic quantum systems. The last decade witnessed an explosive growth of research the superconducting devices for quantum computation (superconducting qubits). However, the decoherence time for these devices remains insufficiently long for large-scale quantum computing. Recently it was predicted [1-4] that the arrays of small Josephson junctions with nontrivial topology may exhibit a novel phase which is characterized by long-range order of pairs of Cooper pairs in the absence of long-range correlations in single-Cooper-pair condensate. Experimental realization of this novel phase can facilitate the fabrication of fault-tolerant superconducting qubits exponentially protected from local noises. Our experiments [5] with small Josephson arrays show that, indeed, the condensate of pairs of Cooper pairs can be observed in the absence of coherence in the single-Cooper-pair condensate. The charge transport in this regime is due to coherent co-tunneling of pairs of Copper pairs, objects with charge 4e. These experiments suggest that even a relatively small prototype device is well protected against magnetic flux variations.

1. L.B. Ioffe and M.V. Feigel'man, Possible realization of an ideal quantum computer in Josephson junction array, Phys. Rev. B 66, 224503 (2002).
2. B. Doucot and J. Vidal, Pairing of Cooper pairs in a fully frustrated Josephson-junction chain, Phys. Rev. Lett. 88, 227005 (2002).
3. B. Doucot et al., Topological order in the insulating Josephson junction arrays, Phys. Rev. Lett. 90, 107003 (2003).
4. B. Doucot et al., Protected qubits and Chern-Simons theories in Josephson junction arrays, Phys. Rev. B 71, 024505 (2005).
5. S. Gladchenko, D. Olaya, E. Dupont-Ferrier, B. Doucot, L.B. Ioffe, and M.E. Gershenson, Superconducting Nanocircuits for Topologically Protected Qubits, Nature Physics 5, 48 (2009).
Host: Robert McDermott
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